1. Technical Field
The present invention relates to an exposure device and an image forming apparatus.
2. Related Art
Heretofore, in a copier, printer or the like that forms images by an electrophotography system, a laser ROS (raster output scanner) system exposure device, which scans light emitted from a laser light source with a polygon mirror, has been used as an exposure device for writing a latent image onto a photosensitive drum. Recently, LED system exposure devices that use light-emitting diodes (LEDs) for light sources have been coming to dominance in place of laser ROS system exposure devices. An LED system exposure device is referred to as an LED print head, or as an LPH for short.
An LED print head is provided with an LED array, in which numerous LEDs are arranged on an elongated support, and a lens array in which numerous graded index-type rod lenses are arranged. In the LED array, the numerous LEDs are arranged to correspond with a number of pixels in a main scanning direction (fast scanning direction), for example, 1200 pixels per inch (i.e., 1200 dpi). Cylindrical rod lenses, typified by SELFOC (registered trademark) lenses, are used as the graded index rod lenses.
In the LED print head, the light emitted from each LED is condensed by the rod lens, and an erect unit magnification image is focused on a photosensitive drum. Therefore, the scanning optical system of a laser ROS system is not required, and a great reduction in size compared to a laser ROS system is possible. Moreover, a driving motor for driving a polygon mirror is not required, which has the advantage that mechanical noise is not produced.
A number of technologies have been proposed in which, in an LED print head, an array of hologram elements is used instead of the rod lenses.
Because electrophotography system exposure devices in which LED print heads use LED arrays are common, these exposure systems are widely referred to as LED systems. However, because light-emitting elements are not necessarily limited to LEDs, hereinafter, an LED system will instead be referred to where appropriate as a light-emitting element array system.